X

n. (also x) (pl. Xs or X's) 1 the twenty-fourth letter of the alphabet. 2 (as a Roman numeral) ten. 3 (usu. x) Algebra the first unknown quantity. 4 Geom. the first coordinate. 5 an unknown or unspecified number or person etc. 6 a cross-shaped symbol esp. used to indicate position (X marks the spot) or incorrectness or to symbolize a kiss or a vote, or as the signature of a person who cannot write.